An earlier thread with a mix of advice on how to build (although without having exact manufacturer instructions), and advice on not building them, lest one screw up a critical component that needs dimensional accuracy:

Very interesting and I did not know that. The reason I was considering making them is because Ive noticed the ones we buy (not sure from where but I know weve boughtt them from UPT directly) have a considerable difference in length...Even 1/4" makes a considerable difference in closing...I had a small collection going at one point and there was about a 1.1" difference between some of them... Most of them measure slightly shorter than spec... This brings me to wonder, do the "approved" riggers also have approval to sell them in bulk to places like Paragear or does everyone who buys them end up with UPT closing loops no matter who the retailer? Its just really annoying to buy 10 loops and 4 of them are too short or too long.